Inhibition of hastelloy b corrosion



Sept; 30, 1958 HASTELLOY B CORROSION RATE HASTELLOY B CORROSION RATEMILS PER 8760 HOURS MILS PER 8760 HOURS R. I... PIEHL INHIBITION OFHASTELLOY B CORROSION Filed Nov. 7. 1955 ik- CALCULATED ASORTHOPHOSPHORIC ACID 10 2O 3O 4O 5O 60 .70 8O 90 100 HO PARTS CUPRIC IONPER MILLION PARTS PHOSPHORIC ACID FIG.1

k- CALCULATED AS ORTHOPHOSPHORIC ACID l l l I I l l l I l 1O 2O 3O 4O 5O6O 7O 8O 90 100 HO PARTS CUPRIC ION PER MILLION PARTS INVENTORPHOSPHORIC ACID FIGZ ROBERT L. P/EHL rim,

ATTORNEYS 2,854,497 i more or HASTELLOY B coanosrorr Robert L. Piehl,Berkeley, Caliii, assignor to California Research Corporation, SanFrancisco, Calif., a corporation of Delaware Application November 7,1955, Serial No. 545,443

4 Claims. (Cl. Mil-683.15)

Constituent: Weight percentage Molybdenum 26-30% Iron 4-7. Silicon 1.0max. Carbon 0.12 max. Chromium 1.0 max. Manganese; 1.0 max. NickelBalance.

Liquid phosphoric acid is recognized as an efficient catalyst forcertain organic chemical reactions, including certain alkylation andisomerization reactions and the polymerization of normal gaseousolefins. For these reactions it is frequently necessary to employreaction vessels, auxiliary equipment and connecting lines that presentmetal surfaces to the phosphoric acid catalyst and thus become subjectto progressive corrosive attack by the acid. In order to minimize suchcorrosive attack, the equipment is desirably constructed of highlyacid-resistant alloys, especially those nickel-molybdenum alloyscontaining less than about 10% of other metals. Among such alloys,Hastelloy B heretofore has been employed with'considerable success, andthe invention will. be particularly described hereinafter as it relatesto this exemplary nickelmolybdenum alloy.

Hastelloy B, developed to resist certain severe conditions of chemicalcorrosion, is so highly resistant to corrosive attack by phosphoric acidthat it is equalled in this respect by few other metals. Notwithstandingthis fact, liquid phosphoric acid, especially at elevated temperatures,high acid ester content, and relatively low acid concentrations, is sohighly corrosive, to Hastelloy B that it can seriously shorten the lifeof equipment fabricated therefrom. This life should be at least fiveyears (and preferably more) to be considered satisfactory from aneflicient process operation standpoint. A five-year life reasonably canbe predicted for the Hastelloy B components of process equipment ofstandard construction if these components do not corrode at a rategreater than about 50 mils per year of continuous operation, i. e.,about onetwentieth inch per 8760 hours. At'this corrosion rate fiveyears would be required for the corrosion to proceed onequarter inch.

. carbonate without producing any significant change in? the catalyticactivity of the acid-copper solution. For

The foregoing maximum tolerable corrosion rate of Hastelloy B in thepast has been a serious limitation on process flexibility, especially inpolymerization processes employing liquid phosphoric acid as apolymerization catalyst. Process flexibility in these processes requiresthat acid concentrations be lowered where necessary to prevent theformation of certain heavier polymer products and that'processtemperatures be raised to increase polymerization efficiency. However,the making of either of these changes results in a marked increase inthe corrosion rate of the Hastelloy B that is exposed to the phosphoricacid, and in a corresponding shortening of the life of the processequipment embodying that alloy. To make both changes even more quicklyshortens the process equipment life, because the combined effect of thechanges on the corrosion rate of the Hastelloy B is cumulative.

Bearing in mind the foregoing conflict between a tolerable Hastelloy Bcorrosion rate and a required flexibility of acid concentration andprocess temperature, it is an object of this invention to provide amethod whereby the corrosion rate of Hastelloy B in contact with liquidphosphoric acid can be maintained within acceptable limits even whenemployed at relativelyv high process temperatures and low acidconcentrations.

I have found by a series of tests, the data from which are set forth anddiscussed hereinafter that, within cer-. tain ranges, the addition ofsmall quantities of cupric ion to liquid phosphoric acid at relativelyelevated tempera-' tures and acid concentrations will cause thecorrosion.

rate of Hastelloy B in contact with the phosphoric acid to decrease,pass through a minimal value and then rise. This result takes place atacid concentrations of about and above (expressed as H PO cupric ion concentrations in the range of from about 2-25 p. p. m., and temperaturessubstantially above 265 F., preferably at least 290 F. By maintainingthe cupric ion concentra- 1 tions at certain acid temperatures andconcentrations'in the corrosion-inhibiting range so found, previouslylimiting acid temperatures may be raised and acid concentra tionslowered to the extent thatthe increase in the corrosion rate of theHastelloy B attributable to those changes equals the reduction incorrosion rate' of the Hastelloy B attributable to the presence of thecupric ion.

It is quite unexpected thus to find that certain concentrations ofcupric ion in the acid will inhibit the corrosion of Hastelloy B, inview of prior art statements that strong oxidizing agents such as copperwill always accelerate the corrosive attack of liquid phosphoric acid onHastelloy B. For example, in Chemical Engineer ing for July 1952, atpage 316 of an article in the Corrosion Forum, it is said, The majorresistance displayed by Alloy B (Hastelloy B) is to reducing conditions.Strongly oxidizing agents are to be avoided. The presence of cupric,ferric, or other oxidizing ions tend to accelerate attack of the alloy.

In the tests reported in Table I the cupric ion was, added as basiccupric carbonate, CuCO .Cu(OH) which. reacted with the phosphoric acidto liberate CO thus-v removing any interfering anions from the solution.Since the amount of copper compound added is very small,

other copper compounds may be used instead of the cupric.

example, copper phosphate (Cu (PO .3H O) or other copper salts may beused.

Data in the following table were obtained in the above- -tions andtemperatures noted in the table.

Calculatedas'orthophosphorlc acid.

' Range of values-obtained intwo or'rnore tests.

The data from the above table 'may be seen in more graphic form, furtherobjects and advantages of the invention will be more apparent,-andthe'following 'discussion of the invention will be moreclearlyunder'st'ood, by reference to the a pended drawings, inwhich:

Fig. l'is a graphical representation comparing the effects of twodifferent temperatures on Hastelloy B corrosion rates at various cupricion concentrations and constant acid concentrations; and 7 Fig. .2 is agraphical representation comparingthe eflects of two different acidconcentrations on'Has'telloy B corrosion rates at various cupric ionconcentrations and constant temperatures.

Referring now to Fig. 1, there shown graphically is the efiect onHastelloy B corrosion rate of the addition to 70% ,phosphoric acid(calculated as orthophosphoric acid) 'of various quantities of cupricion at two 'difi'erent temperatures, viz., 265 F. and 320" F. It may beseen from a comparison of the lower and upper curves that, regardless ofthe cupric ion concentration, but particularly in the absence of cupricion, an increase in. 'teinperature increases the Hastelloy B corrosionrate. Therefore, when a process was heretofore operated at or neartolerable Hastelloy B corrosion rates, a further temperature increase toincrease process efficiency was precluded, because such increase wouldcause the tolerable Hastelloy B corrosionrates 'to be exceeded.

It may also'b'e seen from Fig. 1 that at 'the l'owertemperature of 265F., the addition to 70% acid of progressively greater quantities ofcupric ion over a range of to 50 p. p. m. of cupric ion was accompaniedby a vi'rtually linear increase in the corrosion rate of Hastelloy Brather than by a decrease. Thus, at this temperature (as well as atlower temperatures, where the results are essentially the same) the useof copper ion is to be avoided. However, an examination of the uppercurve in Fig. 1 discloses that totally unexpected results are obtainedon adding progressively larger quantities of cupric ion to the "acid atthe higher temperature of 320 F. First, the curve shows that at thistemperature and zero cupric ion concentration, the above-discussedtolerable Hastelloy B corrosion rate of 50 mils per year is alreadyreached and would be exceeded by a further temperature increase (or adecrease in acid concentration). However, a sharp drop in corrosion rateis obtained with the addition of small quantities of cupric ion.Addition of 5 p. p. m. of cupric ion results in approximately halvingthe corrosion rate, while the addition of 1 0 p. p. m. of cupric iondecreases the corrosion rate to but 17 mils per year, 'a decrease ofapproximately 60% over the corrosion rate prevailing in the absence ofcupric ion. At this point the further addition of cupric ion causes thecurve to pass through a minimal corrosion rate value and take anunexpected upward turn. Thereafter, the addition of cupric ion causes aprogressive corrosion rate increase.

Referring now tov Fig. 2,. there shown graphically is the etfeet onHastelloyrB corrosion rate of the addition of various quantities ofcupric ion to 70% phosphoric acid and phosphoric acid, in 'each casecalculated as orthophosphoric acid, and in each case at a temperature of320 F. It may be seen from a comparison of the upper and lower curvesthat below a cupric ion concentration of about 10 p. p. m., a decreasein acid concentration from 85% to 70% increased the Hastelloy Bcorrosion rate. This is in accord with the general theory, but onlyinsofar as cupric ion concentrations of less than about 10 p. p. m., areconcerned. At concentrations above this amount, the higher acidconcentration results in a higher corrosion rate, an effect which doesnot accord with the general theory.

From the foregoing it may be seen that a marked decrease in thecorrosion rate of Hastelloy B surfaces exposed to phosphoric acid can beobtained, particularly at temperatures above about 290 F. and at acidconcentrations of about 70% or more, by maintaining from about 2 to 25p. p. m., of cupric ion in the solution, with especially good resultsbeing maintained in many cases when the amount of cupric ion maintainedin-the 'solution fallsin therange from about 5 to 15 p. p. m. While thebenefitsof such copper addition are less marked in the case ofphosphoric acids having a concentration of -'or more since such acidshave a relatively low rate of attack on Hastelloy B even in the absenceof copper, nevertheless even here the maintenance in the acid of cupricions within the 2-25 p. p. m. range is quite effective in reducing thecorrosion rate, as evidenced by the data in Table 1 above.

While in the above discussion only commonly used temperatures and acidconcentration ranges have been discussed, those skilled in the art willbe able to perceive numerous other temperature and acid concentrationranges with which the methods of this invention may be usedwithoutdepa'rting from the spirit thereof. All such ranges, and othermodifications and variations of the methods of this invention that willbe apparent to those skilled inthe' art are intended to be embraced bythe following claims.

I claim:

1. In an organic conversion process wherein a Hastelloy B alloy is incontact with liquid phosphoric acid at such acid temperature andconcentration combinations, of at'lea'st 290 F. and 70% (calculated asortho-phosphoric acid), respectively, that addition of some quantitlesof cupric ion to said acid will reduce the rate of corrosive attack ofacid on said alloy, the method of enabling said process to be operatedat a lower acid concentration without an increase in the rate of saidcorrosive attack, which comprises maintaining in said acid an amount offrom about 2 'to 25 parts of cupric ion per million parts of said acid.

2. In an'organic conversion process catalyzed by phosphoric acid atconcentrations of at least 70% calculated as orthophosphoric acid, saidprocess being conducted at temperatures of at least about 290 F.-in aconversion zone having metal surfaces in contact with said phosphoricacid, said metal surfaces comprising a molydenumnickel alloy containing2630% molybdenum, 4 7% iron, 0.02-0'.12'% carbon, not more than 1.0%silicon, not more than 1.0% chromium, not more than 1.0% m'angane'se,and a balance of nickel, the method of inhibiting corrosive attack ofsaid metal surfaces by said acid which comprises maintaining in saidacid between 2 and 25 parts of cupric ion per million parts of saidacid.

3. The method of inhibiting the corrosive effects of liquid phosphoricacid having a concentration of at least 70% calculated asorthophosphoric acid at temperatures above about 290 F. on an alloycontaining 26-30% molybdenum, 4-7% iron, 0.02-0.12% carbon, not morethan 1.0% silicon, not more than 1.0% chromium, not

more" than 1.0% manganese, and a balance of nickel, which comprisesmaintaining in said acid a cupric ion concentration of between 2 and 25parts of cupric ion per which comprises providing in said acid aninitial cupric million parts of said acid. ion concentration betweenabout 2 and 25 parts of cupric 4. The method of inhibiting the corrosiveefiects of ion per million parts of said acid. liquid phosphoric acid atconcentrations of at least 70% calculated as orthophosphon'c acid, andat temperatures 5 r nces Cit d in the file of this patent above about290 F. on an alloy containing 26-30% molybdenum, 47% iron, 0.020.l2%carbon, not more UNITED STATES PATENTS than 1.0% silicon, not more than1.0% chromium, not 2,547,013 Kemp et a1. Apr. 3, 1951 more than 1.0%manganese, and a balance of nickel, 2,653,177 Kemp et al Sept. 22, 1953

1. AN ORGANIC CONVERSION PROCESS WHEREIN A HASTELLOY B ALLOY IS INCONTACT WITH LIQUID PHOSPHORIC ACID AT SUCH ACID TEMPERATURE ANDCONCENTRATION COMBINATION OF AT LEAST 290*F. AND 70% (CALCULATED ASORTHO-PHOSPHORIC ACID), RESPECTIVELY, THAT ADDITION OF SOME QUANTITIESOF CUPRIC ION TO SAID ACID WILL REDUCE THE RATE OF